1. Field of the Invention
The present invention relates generally to improvements in electrical current transfer devices for transferring electrical current between relatively rotatable members, the broad class of such devices generally being referred to as slip rings. More particularly, the invention relates to improved current transfer devices for conducting currents between stator and rotor members of sensitive instruments, such as between the relatively rotatable gimbals of gyroscopic instruments, for example, characterized by consistent current continuity with practically zero friction and coupling torque. Specifically, the invention relates to current conducting or transfer devices employing resilient filamentary conductor loops which are compressed to predetermined preloads between concentric, coplanar, radially spaced shaped conductor ring contact surfaces on the relatively rotatable members which loops are self captured by and roll on these surfaces upon relative rotation in the presence of any misalignments between the rings or movement of the loop in a vibratory and shock environment while imparting substantially zero torque between the rotatable members.
2. Description of the Prior Art
Rolling electrical conductor assemblies are not broadly new and have heretofore been proposed for use in place of the more conventional slip ring and brush assemblies. The present inventors are aware of two such rolling type conductor assemblies and these are disclosed in U.S. Pat. Nos. 2,467,758 and 3,259,727. Also, the present inventors are unaware of any adoption of the assemblies disclosed in these patents by industry in general and particularly by manufacturers of precision sensitive instruments such as gyroscopic instruments. The probable reason is that none of the contact assembly configurations disclosed in these patents are suitable for such applications. In this specification "contact assembly" or "contact apparatus" may sometimes be used instead of "conductor assembly", it being understood that they are used synonymously.
As is well known to those skilled in the gyroscopic arts, slip rings and "hair pin" brushes supported in brush blocks have been used for many years for conducting electrical power and signal currents across the relatively rotatable gimbal axes of gyroscopes. While these have been generally satisfactory, they have been plagued with both manufacture and service use problems, causing fairly high removal rates for repair and overhaul. These assemblies are extremely delicate and require high skill in assembling and time consuming adjustment to achieve a preload consistent with minimum sliding friction in a vibration and shock environment. Also, since they are normally exposed during repair and overhaul of the gyroscope, they are subject to being damaged during handling. In service, especially aircraft service, such gyroscopic devices operate in a vibratory environment and since sliding contact exists between brushes and slip rings, friction polymers tend to build up causing electrical shorting and/or open circuits thereby requiring removal for cleaning and/or replacement; again a delicate, time-consuming and costly operation. More importantly, in autopilot systems which derive aircraft body rates from displacement gyroscopes by differentiation of the gyro displacement signals, electrical noise inherent in this type of slip ring assembly is effectively amplified, rendering the rate signal undesirably noisy and requiring heavy filtering thereby detracting from the rate signal quality. Also in digital encoders, for example, conventional slip rings can produce objectionable digital noise. In addition, as appreciated by those skilled in gyroscopics, slip ring and brush assemblies reduce the long term accuracy of gyroscopes because of the relatively high friction-induced torques produced by the usually large number of slip rings and brushes required in modern electrical gyroscopes.
With the foregoing in view, it will be appreciated that devices for conducting current between relatively rotating members of sensitive instruments, such as across the gimbal axes of gyroscopic instruments, should desirably exhibit the following desirable properties: substantially zero friction and coupling torque; relatively consistent current conduction even in a shock and vibratory environment; long reliable life; low cost of manufacture and assembly; and no vibratory sliding friction contact thereby eliminating friction polymer buildup.
The rolling electrical current conducting devices as disclosed in the above-mentioned prior art patents, while perhaps suitable for some applications, (although the present inventors are unaware of any general application in industry of either of these patented devices) are unsuitable for use in apparatus requiring low friction and coupling torques and capable of producing self retention forces without introducing variable coupling torques between the two relatively rotating members. For example, in the first of the above patents, U.S. Pat. No. 2,467,758, a roller band conductor is disclosed for application as a "slip ring" for an alternating current motor and as a rolling contact for an electrical switch potentiometer or rheostat. It will be noted that in all applications suggested by this patentee, friction or coupling torques imposed by the roller band itself together with its retaining mechanism is clearly not a design consideration at all since in all cases one contact member is driven from a mechanical power source. In many applications, for example, sensitive instruments such as gyroscopes, any friction imposed by the electrical contact devices results in undesired torque on the supported member thereby producing undesired drift or precession and reducing the gyroscopes effectiveness as an accurate, long term reference. Also, in this prior patent, the roller band is not self-captured or self-retained in its orbital path between the conductor rings but requires retaining flanges or pin and hole retaining arrangements. Such flanges, pin-and-hole arrangements and the like are completely unsatisfactory in many applications because of the high friction torques and the variable coupling torque magnitude produced by the bands contacting or rubbing against the flanges or pin-and-hole surfaces as it rotates. If a number of circuits are involved, this high and variable torque is correspondingly multiplied making these configurations wholly unsuitable for low torque applications. Also in this patent, the ratio of the free loop diameter to the radial distance between the inner and outer conductor members is very large so that when the loop is compressed into the radial gap, the loop is highly distorted which results in coupling torque hysteresis and premature metal fatigue and rupture. Also, such distortion may produce buckling and further non-uniform torque. Thus an assembled loop which is highly distorted is not suitable in applications where substantially zero coupling torque is desired or required.
The second of the above-mentioned prior art patents, U.S. Pat. No. 3,259,727, discloses a flexible, rolling element current transfer device in which current transfer characteristics are stated to be improved over that of the first of these prior art patents. This improved current transfer characteristic is stated as being accomplished by making the conductor rings on the relatively movable members in the form of a deep or acute "V" whereby the rolling contact element wedges itself into the "V" groove providing a wiping action to assure good electrical contact. This patent employs a small diameter spring closed upon itself to form a torus, the diameter of which is very large compared to the radial distance between the "V" grooves and therefore, when assembled forms a highly distorted or rolling element which wedges itself into the steep "V" walls hence producing high torque coupling. A flat band is disclosed as an alternative but, like the spring torus, wedges itself between the steep sidewalls of the "V" groove. It is quite evident that the device disclosed in this prior art patent is entirely unsuitable for use in applications which require substantially zero friction and coupling torques to be imposed on the supported rotatable member by the current transfer assembly because of the substantial wiping or rubbing friction and uncompensated bending moments generated as the coil or band enters and leaves the "V" grooves.
From the over-all disclosures of the above prior art patents, none of the rolling contact configurations can exhibit low friction and coupling torque. Furthermore, the above prior art patents disclose no methods, techniques or apparatus for assembling the loops in the radial space between the conductor rings.